The present invention relates to a power brake unit for automative vehicles comprising a brake valve which is adapted to meter pressure fluid from an auxiliary energy source into a booster chamber, the booster chamber being isolated from an intermediate pressure chamber by an axially displaceable booster-piston arrangement including a first booster piston and a second booster piston. The second booster piston is axially slidable relative to the first booster piston, the sliding motion being limited by a stop, and the second booster piston is held at a maximum axial distance from the first booster piston in the actuating direction of both booster pistons by a means. The intermediate pressure chamber, on the one hand, is bounded by a master-cylinder piston or an intermediate piston coupled thereto, and a hydraulic transmission ratio effective between the brake pedal and the master-cylinder piston is disconnectible dependent on the pressure of the auxiliary energy source.
Such a power brake unit is disclosed in U.S. Pat. No. 4,263,784, issued Apr. 28, 1981, and includes an auxiliary piston rigidly coupled through a yoke to a first booster piston and is in turn guided in a cylindrical bore of the second booster piston.
The end face of the auxiliary piston and parts of the cylindrical bore combine to form a pressure chamber which is in communication with a pressure source via a radial bore of the second booster piston, via an annular chamber at the first booster piston and via a pressure fluid port. In the inactivated off position of the power brake unit, a booster chamber is connected to a return line through a valve integrated in the first booster piston, as a result of which the booster chamber is depressurized.
Upon the application of a force on the brake pedal, first a return port of the brake valve is closed and then the pressure fluid source is connected to the booster chamber. The pressure build-up in the booster chamber conditions a joint movement of both booster pistons, with the auxiliary piston which is partly guided in the second booster piston shifting a pressure cushion ahead of itself. The pressure fluid enclosed by the second booster piston is thus fed into the intermediate pressure chamber causing the intermediate piston to move away from the first booster piston and to actuate the master-cylinder piston.
If the auxiliary energy fails, the second booster piston remains in its position while the pressure fluid in its inside bore is supplied through the auxiliary piston to the fluid source. In a like case of failure, an actuation of the master-cylinder piston is safeguarded by a mechanical abutment of the first booster piston on the intermediate piston with the hydraulic transmission ratio being eliminated.
In the area of the second booster piston, the construction of the prior art power brake unit described above entails comparatively great expenses, since the bore accommodating the second booster piston, the inside bore of the second booster piston as well as the auxiliary piston guided therein are required to have a fine surface quality which demands costly machining operations.